Invention relates to a receiver for receiving frequency modulated signals and having a controlled acquisition band.
In certain phase-locked loop receivers, it must be possible for performance levels to be maintained in an environment having a very high level of jamming; i.e. with interference signals whose frequencies are very close to the frequency of the useful signal and whose power levels are very high (e.g..+-.2.6 MHz with a jammer-to-signal ratio J/S.gtoreq.50 dBp). For some of these receivers, it must be possible for such performance levels to be maintained in a space environment, and this poses problems linked to temperature and to ageing, etc.
Such performance levels are optimum when the signal which is applied to the demodulation circuit is almost perfectly centered in the intermediate frequency (IF) filter. It is therefore necessary to use a system which automatically recenters the received signal on the center frequency of the IF filter.
French Patent Application FR 2 596 220 describes a prior art frequency demodulator including a channel selection circuit, an intermediate frequency selection circuit, and an automatic frequency control circuit. That prior art frequency demodulator is described below with reference to FIG. 1.
Such a demodulator suffers from a certain number of drawbacks:
it is protected from very high level jammers only by the quality of the intermediate frequency filter; and PA1 it includes three servo loops each of which requires passband adjustment. PA1 a frequency translation circuit including an intermediate frequency filter; and PA1 a recentering loop for recentering the signal received in the filter. PA1 an amplifier; PA1 an image frequency rejection filter; PA1 a mixer which receives a signal from a VCO (Voltage-Controlled Oscillator); PA1 a first controlled gain amplifier; PA1 an IF pass-band filter; PA1 a second controlled gain amplifier; and PA1 a coupler which is connected to an IF level regulating circuit which controls the two controlled gain amplifiers. PA1 a mixer; and PA1 a loop filter amplifier whose output is the output of the receiver; an oscillator being connected between this output and an input of the mixer. PA1 a reference oscillator whose frequency is (IF/K) 1/K times the frequency (IF) of the IF filter. PA1 a first programmable frequency divider; PA1 a phase/frequency comparator which, at a second input, receives the signal from a coupler positioned at the output of the oscillator of the phase-locked loop frequency demodulator, via a second programmable frequency divider; PA1 a loop filter; and PA1 an electrically controlled switch whose output is connected to the VCO of the frequency translation circuit.
U.K. Patent Application GB-A-2 079 552 describes a double phase lock loop system for receiving frequency modulated signals, which system includes a frequency translation circuit having an intermediate frequency filter and a decision circuit for deciding to close the phase-locked loop. The decision to close the phase-locked loop of the apparatus is taken solely on the basis of locking being detected.
Such a demodulator also suffers from insufficient protection against jamming, such protection being provided solely by the intermediate frequency filter. Furthermore, no provision is made prior to closing the phase-locked loop to discover the nature of the signal which has caused synchronization to be acquired, thereby allowing the loop to be closed indiscriminately on a jammer or on a signal as soon as locking is detected.